The present invention concerns the preparation of symmetrical and asymmetrical carbonates of the general formula I 
wherein R and Rxe2x80x2 are the same or different and signify a straight-chained or branched alkyl group with 1 to 10 C-atoms, a benzyl group unsubstituted or substituted with up to three C1-C4-alkyl groups, C1-C4-alkoxy groups, halogen atoms, with a cyano group, a nitro group, a trifluoromethyl group or an alkoxycarbonyl group with up to 4 C-atoms, an aralkyl group or an alkenyl group. The term aralkyl group includes a lower alkyl radical with 2 to 10 C-atoms, wherein up to two H atoms are replaced by phenyl groups, which again can be substituted with a C1-C4-alkyl group, a C1-C4-alkoxy group, a cyano group, a nitro group, a trifluoromethyl group, an alkoxycarbonyl group with up to 4 C-atoms or with up to three halogen atoms. The term alkenyl designates an unsaturated hydrocarbon radical with up to 5 C-atoms.
Organic carbonates play an important role as solvents, as intermediate products for numerous syntheses and as products for special fields of use, e.g. in agricultural chemistry or medicinal chemistry (Ullmann""s Encyclopedia of Industrial Chemistry, 5th edition, Vol. A5, p. 197, 1986; KIRK-OTHMER, Encyclopedia of Chemical Technology, 3rd ed., Vol. 4, p. 766, 1978; Abbas-Alli G. Shaikh, Chem. Rev. 1996, 96, 951-976).
The preparation of open-chained organic carbonates can e.g. take place (i) from phosgene and hydroxy compounds, (ii) from haloformic acids by reaction with hydroxy compounds, (iii) by alkylation of alkali metal carbonates, (iv) by transesterification of carbonic acid diesters or (v) from carbon dioxide and alcohols under pressure in the presence of catalysts or, however, according to other special processes (H. Hagemann, HOUBEN WEYL, E4, p. 65, 1983; Abbas Alli G. Shaikh, Chem. Rev. 1966, 96, 951).
Processes for the preparation of organic carbonates which avoid the use of the highly toxic phosgene which can use carbon dioxide present and start from simple raw materials are of especial interest from industrial as well as organic-preparative point of view. Stimulated by a work of Ken J. Butcher for the preparation of carbamates from amines and carbon dioxide (Ken J. Butcher, Synlett 1994, 825), it was investigated by us whether alcohols of the general formula II, with use of carbon dioxide, caesium. carbonate and alkyl or aryl halides of the general formula III,
Rxe2x80x94OHxe2x80x83xe2x80x83II
Rxe2x80x2xe2x80x94HALxe2x80x83xe2x80x83III
whereby R and Rxe2x80x2 possess the above mentioned meaning and HAL stands for chlorine, bromine or iodine, can be converted into organic carbonates of the general formula I (scheme 1):
On the basis of the lower nucleophilia of the OH group in alcohols in comparison with the NH2 group in amines and on the basis of the special methods described in the literature for the preparation of carbonates from carbon dioxide (Abbas-Alli G. Shaikh, Chem. Rev. 1996, 951, 966), a synthetic access to carbonates with use of the system carbon dioxide/caesium carbonate at low temperatures was not to be expected.
Surprisingly, however, it was found that organic carbonates of the general formula I can be prepared under very mild and preparatively simple conditions in the presence of alkali metal carbonates, especially caesium carbonate, from alcohols of the general formula II and alkyl or aryl halies of the general formula III. For this reaction, surprisingly no further catalyst is necessary. The preparative procedure is as follows:
The alcohol and a 2 to threefold molar excess of caesium carbonate are placed in a suitable dipolar aprotic solvent, such as e.g. dimethylformamide, acetonitrile, dimethylacetamide or N-methylpyrrolidone, at room temperature. With good stirring, carbon dioxide gas is now passed at room temperature, with exclusion of moisture, for 4 to 6 hours into the reaction mixture (about 5 bubbles/second). The carbon dioxide is hereby produced by allowing dry ice to evaporate which is present in an Erlenmeyer flask which is connected with the reaction vessel via a gas inlet pipe. One now adds to the reaction mixture in one portion 1 equivalent (referred to the alcohol) of the alkyl or aryl halide in question of the general formula III, dissolved in a little solvent, passes further carbon dioxide in for 1 hour, again adds thereto 5-100%, preferably 10%, of the original amount of alkyl or aryl halide and then closes the reaction vessel. With closed reaction vessel, one now stirs further for 24 hours to 3 days at room temperature. Thereafter, one pours the reaction mixture on to water, extracts the product with ethyl acetate and purifies the so obtained raw product with the methods usual in preparative organic chemistry, e.g. by chromatography or crystallisation. Preferred solvent for the described reaction is dimethylformamide.
The reaction conditions are very mild, there are tolerated many functional groups, such as e.g. the double bond, the nitro group, the alkoxycarbonyl group, the cyano group, halogen groups and alkoxy groups on aromatics. The starting materialsxe2x80x94alcohols and alkyl and aryl haliesxe2x80x94are simple to prepare and are commercially available in large number. The conditions for the working up of the reaction are very easy to produce. With the assumption that caesium carbonate can again be prepared from the extracted aqueous residue, the method is suitable to bind gaseous carbon dioxide on to simple commercially available starting materials, such as alcohols and alkyl or aryl halides and thereby to produce valuable, energy-rich intermediate products. In this sense, the said process is a valuable addition to an environmentally friendly chemistry.
Because of the simplicity of the process, the method of procedure is also suitable as basis for a high throughput synthesis. For this purpose, in a carbon dioxide gasification apparatus which contains DMF solutions of corresponding alcohols, would have to be gassed with CO2 for some hours. Thereafter, the corresponding alkyl or aryl halides are to be dosed thereto, the vessel to be closed and to be stirred for 24 hours to 3 days at room temperature. Thereafter, the carbonates formed are to be isolated in simple manner.